registry: check for running as root in clustering mode

registry: check for running as root in clustering mode

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14787
RN:  net conf list crashes when run as normal user

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Tue Aug 17 11:23:15 UTC 2021 on sn-devel-184

(cherry picked from commit 4809f4a6ee971bcd9767839c729b636b7582fc02)

Autobuild-User(v4-13-test): Jule Anger <janger@samba.org>
Autobuild-Date(v4-13-test): Mon Sep  6 10:16:27 UTC 2021 on sn-devel-184